For a couple of the same reasons Kurt mentions. Mixed groups tend to have a lot of wasted time. And a dedicated jump tender just sits there not providing much of anything (though I suppose you could have a tender/tanker).
I usually run a survey squadron with 5 ships. No weapons/scanners/defenses. They survey the system and move on to the next.
I usually end up with my specialized designs (survey ships, freighters, harvesters, colonizers) having a jump drive. The warships will have one or more assault tenders (max squadron size/distance). The warships don't mount the drive for size/speed/more offense/defense in that spot.
Of course, if the tender is blown the war fleet is hosed.
Risky, having unarmed survey groups with no defenses or even sensors. I usually at least give my survey units decent passive scanners, so they might have a chance to see something coming and get out quick.
Of course, my current approach, with the Jovians, isn't the best. Their 1st Survey Group, before it got hosed by a precursor, was a Vanguard Class Jump/Geo-Survey ship, a Cortez Class Geo-Survey Ship, and a Tribal Class DDE. The shortcomings in this configuration are numerous, as the recent hosing illustrated. The Vanguard was the first ship to detect the precursor, and if it had been destroyed the other two ships would have been stranded. The only fact that saved the Vanguard was that they had better sensors than the precursor and detected them first, which allowed them to retreat and bring up the other ships. The DDE doesn't really have anti-ship capability, as it only has three 10 cm lasers, which raises the question of why it was there at all. The explanation for that was that the Jovians couldn't spare a larger ship, but felt that they had to have a warship along.
I've been doing a lot of thinking about this subject lately, because of the experience of the 1st Group getting 2/3's destroyed. While their deployment strategy was faulty, the reality was that against a precursor, there was little the Jovians could do. This is kind of an interesting situation, I think.
One of the possible alternatives I came up with is to have all survey ships equipped with jump drives and deploy them independently, on solo missions. That way, if they are lost, no other ships are lost with them or stranded because they don't have their own jump drives. Of course, that also raises the issue of loss of knowledge. If a grav-survey ship goes out and explores a system (system 1A), discovers, say, two warp points, probes both, moves into one of the systems (system 1B) and surveys it and then begins a survey in one of the systems discovered beyond the second system (system 1C), and then is destroyed, that is a lot of information lost and the homeworld will have no way of knowing where to look. Worse, given the fact that missions often last for several years, the homeworld would have no way of knowing anything was wrong until long after it was too late to do anything about it.
Kurt